Taking this thread from Smogon. Cuz I can, and Serebii might bring something up that they haven't.
Well, cores are one of the main parts of teambuilding. Some are also effective in checking playstyles or in checking pokemon.
What works good for you? What stuff works great with X and what does Y do that complements X?
I'll start with this Gravity core that sucks, but could be used with good support:
Starmie @ Leftovers
EVs: 252 HP / 24 Def / 8 SpA / 224 Spe
[Scald] / [Psyshock]
Landorus - T @ Choice Scarf
EVs: 8 HP / 252 Atk / 248 Spe
Forretress @ Leftovers
EVs: 252 HP / 204 Def / 52 SpA
[Rapid Spin] / [Stealth Rock]
Well, Starmie allows Lando-T to spam Earthquake. I find that CB is slightly ineffective, but CS still works. Gravity's effect, as you most know, allows flying/levitating pokemon to be affected by EQ and by Spikes, which explains Forry's use. Forretress can also beat Haxorus locked into Outrage/Salamence without Fire Blast/Dragonite with SR and other stuff, whilst spinning in their faces. Pain Split works for recovery. SR can be used if you want, but Heatran can provide that if you like. Starmie and Forry resist everything but Rock/Flying/Electric/Ground, so Lando helps by neutralising Ground/Electric and also neutralises Rock and Flying, is Rock is mainly physical. Flying is sorta mixed, so meh. Be aware of Keldeo if you don't run Psyshock, as it can beat all of these one-on-one with Scarf or Specs. Amoonguss helps here as a team mate, so consider it.